I think you are conflating 2 issues; apt-get install adds a tool to the base OS system - this requires elevated permissions. npm is such a tool that, once added, allows you to add nodes either globally (available to all users) or locally (just one user). You shouldn’t need sudo to use npm to install nodes (either locally or globally). Note the exception is that you need elevated permissions when using npm to install npm (and no I don’t understand that either…).
Using the approved install script, I believe, Node-Red will generally be installed using npm without elevated privileges.
Interestingly, despite what the NR docs say, using the script to install NR results in NodeJS v10 being installed (according to the text in the script itself)!